1. Field of the Invention
This invention relates generally to illumination devices, especially light bulbs, and more particularly to illumination devices in which a filament is made incandescent so as to provide light, and for which a principle design element has been the attainment of a longer useful life.
2. Background Information
Incandescent light bulbs can become non-functional, or "burn out," as a result of breakage or other loss of structural integrity. Such bulbs include metallic filaments that become broken, or there may occur a rupture in the hermetic seal by which the lead-in conductors that provide power to the device are passed into the interior of the bulb. Typically, such breakage or loss of integrity results from simple evaporation of the filament material and a resultant loss of mechanical strength. Breakage may also result from heat stress, both from heat cycling in being turned on and off, and from temperature gradients established at the juncture of various internal structures with the glass or glass-like envelope that surrounds those internal parts and provides the support required to hold those parts in position.
To counteract such filament evaporation, such bulbs may include, along with a filling of inert gas such as argon, a small amount of halogen or compounds thereof, the latter acting to re-deposit evaporated filament material back onto the filament at the hottest parts thereof. While this procedure is quite effective in permitting hotter filament temperatures and hence more efficient light emission, a consequence of having the halogen gas present is that filament material is "scavenged" thereby from the colder ends of the filament near to the lead-in conductors and the seal with the surrounding glass envelope, whereby such colder ends similarly may become weak and eventually break.
Another source of bulb failure involves the interface between the glass envelope and the lead-in conductors. For example, U.S. Pat. No. 3,420,944 issued Jan. 7, 1969 to Holcomb describes a lead-in conductor having a thin intermediate foil or ribbon portion made of a refractory metal such as molybdenum at the point of the seal. Such metal is subject to oxidation at its outer end and ultimately to breakage as a result of the infusion of oxygen from the surrounding atmosphere into the seal, hence one solution to the problem has been to provide to the foil a coating of an oxidation-resistant material such as chromium. However, such coatings have been found to undergo chemical reaction with the internal halogen atmosphere, hence to avoid that occurrence Holcomb limits the application of the oxidation-resistant material to an outer segment of the foil, the inwardly extending uncoated molybdenum being unaffected by the interior halogen atmosphere.
In addition to chemical attacks on such intermediate ribbon portions, as was previously noted active halogens such as bromine also cause corrosion of the filament coils themselves near the ends of the filament, as a result of sharp temperature gradients both between adjacent coils of the filament and between the filament and a connecting spud that attaches the filament to the lead-in connectors. In U.S. Pat. No. 3,431,448 issued Mar. 4, 1969 to English, that effect is sought to be minimized or eliminated through use of a structural design wherein sharp temperature gradients are reduced. U.S. Pat. No. 4,568,854 issued Feb. 4, 1986 to Westlund et al., uses a particular ceramic base for improved heat dissipation in a type of high temperature tungsten halogen lamp.
Yet another cause of filament breakage lies in a "sagging" effect whereby an initially horizontally disposed filament having the usual coiled structure will become elongated upon being incandesced, the center portion thereof will sag downwardly from the effect of gravity, and the resulting curvature in the filament may ultimately bring adjacent turns of the coils thereof into contact so as to short out, or contact may similarly be made between a coil and a mounting structure. Even in the absence of such breakage, such sagging serves to move downwardly the central source of light from the bulb, thereby requiring adjustment in the position of the bulb when it is used as a light source in a focused system of lenses. As described in U.S. Pat. No. 3,789,255 issued Jan. 29, 1974 to Sell et al., inclusion within the filament material of dopants in the form of alkali silicates, or in the Sell et al. patent itself of means for producing helium-filled bubbles within the filament, will result upon incandescence of the filament in the growth of elongated and interlocked grains or crystals that are axially disposed and serve to minimize sagging.
U.S. Pat. No. 4,451,760 issued May 29, 1984 to Griffin et al. addresses the issues of filament sag and halogen corrosion through the introduction into the envelope of a quantity of copper, e.g., as one of the lead-in wires, a plating on the lead-in wires, a separate copper insert, or a coating on the filament. One source of filament sag is understood to lie in grain boundary slippage within the filament material, particularly in halogen lamps that will have present some amount of gaseous oxygen, since such slippage is thought to be facilitated by free oxygen. That oxygen is also thought to exacerbate metal corrosion by the halogen gas. The copper serves to remove oxygen and thereby minimize both grain boundary slippage and corrosion.
In U.S. Pat. No. 4,449,398 issued Feb. 12, 1985 to Munroe, an elongate incandescent bulb is described that has relatively massive terminals at each end thereof, and one or more elongate helical coils, supported on a stiff refractory central element, extending therebetween. This structure permits use of filament coils of a size larger than is customary and is thus less susceptible to breakage.
As opposed to such direct energy input to a filament, fluorescent bulbs of the type described in U.S. Pat. No. 3,987,335 issued Oct. 19, 1976 to Anderson operate not by means of a heated filament but rather by the fluorescence of a phosphor that has been excited by radiation from a contained, ionized gas. As described by Anderson, previous efforts to provide excitation of the contained gas so as to produce that ionization have included coupling electrical energy thereinto by means either of ordinary induction using an electrical air transformer or by electromagnetic induction, i.e., an rf energy source, but such efforts have proved to be too inefficient and also become sources of possibly dangerous rf radiation. It is also known to use rf fields for the purpose of "flashing" a "getter," i.e., a piece of magnesium or the like within the bulb is brought to incandescence by an rf field so as to react with residual gases left within an evacuated bulb and thereby effectively remove the same.
Efforts based upon the use of iron or ferromagnetic transformer cores have likewise proved to be too inefficient, i.e., at the frequencies required for useful energy transfer to the gas the eddy current heating losses occurring in such cores become too great, resulting not only in a loss of energy but also in creating unacceptable heat levels within the bulb. The Anderson patent describes a device in which ionization in the gas is induced by a transformer that is only partially contained within the bulb envelope, the portion of the transformer that is open to the atmosphere serving as a means for cooling of the transformer as a whole.
Lamps that provide fluorescence energy by means of a spark discharge within a gas are described in U.S. Pat. No. 4,187,446 issued Feb. 5, 1980 to Gross et al. and in U.S. Pat. No. 4,311,942 issued Jan. 19, 1982 to Skeist et al. Both such patents employ ballast designs that employ diverging magnetic fields which serve to expand the arc volume for increased gas excitation while also limiting the arc current. U.S. Patent No. 4,855,635 issued Aug. 8, 1989 to Grossman et al., describes the use of permanent magnets in manipulating the arc shape.
From the foregoing, it is apparent that heating of a filament to incandescence by direct electrical connection to an external current source must necessarily involve numerous technical problems arising principally from the presence of heat gradients in passing from inside to outside of the bulb envelope and within the filament itself. Sagging of that filament introduces another cause of breakage. Fluorescent bulbs avoid such filament problems, and moreover provide light with substantially greater efficiency than do incandescent bulbs. However, the fluorescent bulb introduces its own inefficiencies, commencing with the need to achieve a transformation of electrical energy from a conducting environment to a gas; secondly, ionization of that gas; thirdly, ionic de-excitation to provide radiation; fourthly, application of that radiation to the excitation of a phosphor; and finally, emission of light from that phosphor. In particular, the process of exciting the gas by means either of inductive coupling or an arc discharge must be inherently inefficient, given the volume-extended and spaced-apart nature of any gas relative to any particular means for exciting the same. Such inefficiency is exacerbated by the fact that energy absorption by a gas is at least quasi-quantized in nature, and the relatively crude, macroscopic methods of inductive or arc excitation cannot take account of the most efficient energy absorption profile of the gas as defined, e.g., by its frequency-dependent Einstein absorption coefficient. Such processes may be improved by the use of appropriate magnetic fields to control and shape the excitation process, but yet it remains as a substantial barrier to the efficient production of light. What is needed and would be useful, therefore, is a method and apparatus by which a filament within an evacuated envelope could be brought to incandescence while avoiding the problems of heat gradients inherent in the use of lead-in conductors. It would also be useful if means could be provided by which sagging of the filament could be avoided. No such method and apparatus being otherwise available, they are now provided by the present invention.